Preheat the oven to 325F.
In a 6.5 inch cast iron skillet pan over low heat, add butter and stir until melted. Add brown sugar and granulated sugar. Keep stirring until smooth, do not burn.
Remove the pan from the heat, set aside and chill for at least 15 minutes.
Add the vanilla and egg into the skillet pan, stir until just incorporated for about 10 seconds. Stir in the all-purpose flour, baking soda, and salt. Mix well.
Stir in the chocolate chips, chocolate chunks, and optional nuts until combined. Top with the marshmallow.
Bake for about 18 minutes (the center will be soft). Do not overbake. Remove from the heat and cool on the wire rack before serving.
Serve with homemade chocolate coffee ganache. (recipe below)